Stress-stimulated luminescent materials which emits light in response to internal stress caused by a mechanical effect such as friction, shearing, impact, and vibration have conventionally been proposed. Known examples of the base material of such stress-stimulated luminescent materials include aluminate, silicate, and semiconductor found in nature. More specifically, for example, the inventors of the present invention have proposed (1) a high-luminosity stress-stimulated luminescent material made up of at least one type of aluminate of a non-stoichiometric composition and including a material which includes lattice defect causing luminescence when a carrier excited by mechanical energy returns to the ground state. A specific example of such a material is MxAl2O3+x (where M is Mg, Ca, Sr, or Ba, and 0.8<x<1). (For example, see Document 1: Japanese Laid-Open Patent Application No. 2001-49251; published on Feb. 20, 2001.)
The inventors have also proposed (2) a stress-stimulated luminescent material whose base material is an oxide constituted by a compound represented as MN2O4 (where M and N are metal elements selected from a group of Mg, Sr, Ba, and Zn and a group of Ga and Al, respectively). (See, for example, Document 2: Japanese Laid-Open Patent Application No. 2002-194349; published on Jul. 10, 2002).
The inventors have also proposed, as an example of a manufacturing method of a high-luminosity stress-stimulated luminescent material, a method of manufacturing aluminate by a sol-gel process, the aluminate being represented by a general formula MxAlyO2x+3y/2 (where M is an alkaline earth metal, a transition metal, or a rare-earth metal, x and y are integers, and the alkaline earth metal as M is a metal element selected from a group of Mg, Ca, Ba, and Sr). (See, for example, Japanese Laid-Open Patent Application No. 2002-220587; published on Aug. 9, 2002).
The conventional stress-stimulated luminescent materials can effectively emit light by mechanical stress, but the light emission luminescence is sometimes insufficient. The applications and uses of the stress-stimulated luminescent materials are therefore limited.
More specifically, under a circumstance where the light intensity of outside light is relatively low, the above-described stress-stimulated luminescent materials can perform stress-stimulated luminescence with an intensity sufficient for visual observation. On the other hand, in case where the light intensity of outside light is as high as daylight, i.e. under a fairly bright circumstance, the light emission luminescence of the stress-stimulated luminescent material may be relatively low and not sufficient for visual observation.
Strong luminous intensity sufficient for visual observation under a bright circumstance is required to broaden the range of uses and applications of luminescent materials. A stress-stimulated luminescent material which can achieve such strong light emission cannot be practically obtained by conventional techniques.